The invention relates to switching power supply (SPS) receiving AC line voltage or DC voltage, particularly for power supply systems requiring high output power, high efficiency and high power factor.
Conventional SPSs convert AC energy source, e.g. line, into DC voltage. In particular, the AC voltage is rectified and further applied to an input capacitor. Charging the capacitor causes inrush current and surge currents every peak of the AC voltage. Pulse width modulation (PWM) is used which results in numerous problems including high circuit complexity, stability problems and significantly higher effective switching frequency. Numerous interference suppressors and protection circuits are inevitable. Power factor correction circuits are employed which further increase complexity and decrease efficiency.
Power factor correction circuits employ a boost switching circuit. Rectified line voltage is applied thereto. Therefore, switching frequency or PWM varies in a large range. The circuit is practically inoperative near line voltage crossover. The voltage provided by the boost circuit is inherently greater than the peak of the line voltage. This value is especially large e.g. in European countries. A troublesome mechanical switch for reducing capacitor voltages is employed. The power factor correction circuit operates as separate input unit and is excluded from the power conversion itself. In particular, the correction circuit must sustain a full power of the SPS. Moreover, the inrush current is inevitable.
Power factor well defines performance of an SPS. It is a ratio of the SPS output power over input power. The input power is input AC voltage multiplied by RMS input current. The power factor is often specified only for the correction circuit. In fact, the power factor depends on efficiency of the entire SPS. An exemplary approach to a high efficiency SPS is disclosed in the abovementioned U.S. Pat. No. 4,736,286 entitled "Switching Power Supply" dated 04/05/88, by the same inventor. The SPS disclosed therein converts AC signal into AC and/or DC signal while employing a minimum number of switching and inductive components. In particular, FIG. 4a embodiment disclosed in U.S. Pat. No. 4,736,286 employs a single inductive component and single capacitor which is not coupled to the SPS input.